It is all obvious or trivial except…

Hmmm

A young “homeless planet”, up to seven times the size of Jupiter and with no gravitational ties, has been spotted by scientists for the very first time.

That must be up around the limit for planet size isn\’t it? For at some point you don\’t get a planet, gravity leads to a star?

Or, given that we\’re talking about something very young here (120 million years old apparently) and thus something made out of old stars with much more of the heavier elements therefore, does that planet/star size change dependent upon said composition?

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30 comments on “Hmmm”

If you’d asked me 20 years ago, when I hadn’t too long stopped being an astrophysicist, I might have had this at my fingertips. As it is, the first source I have is the venerable (c1970) “Habitable Planets for Man” which indicates that the threshold is somewhere between 3 and 30 Jupiter masses. And yes, the presence of metals (elements above hydrogen, in the jargon) will change the details, but memory fails me after that.

I’m in much the same position as steve. 20 years ago, with my newly minted degree, I could have told you, but it’s all a bit hazy now. Self-gravitation eventually leads to pressures and temperatures at the core which cause fusion to begin. Then you have a star. But, the presence of heavier elements than hydrogen makes a HUGE difference, and once you get above iron the whole process stops working.

Yes, the big problem with heavier elements is that if the proto-star forms non-catastrophically, they will gather in the middle and the hydrogen will be forced to lower pressure regions and therefore much less likely to start fusing.

Just as a correction – although iron is the most nuclearly stable element, strictly the process stops with zinc, leaving radioactive nickel-56 as the highest mass-produced element in the silicon burning sequence …

” Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars or stellar remnants are “planets” (no matter how they formed). The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System.

[Which is defined as “enough to clear its orbit for a full planet, or “enough for a rocky body to become spherical under the influence of its own gravity” for a dwarf planet]

Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not “planets”, but are “sub-brown dwarfs” (or whatever name is most appropriate).

So, technically, it’s not a planet, but a sub-brown dwarf. Or, as the paper calls it, an “IPMO” (interstellar planetary mass object).

There certainly seems to be some disagreement here. What you need is an expert to give you a definitive ruling but they’ll need to be of great intellect, completely conversant with the subject & totally without prejudice.

Under current IAU definitions, only a small number of bodies in our own solar system in near-circular orbits which have been “cleared” are planets. Anything else planet-like, including this body, is an exoplanet.

Scuse the bloody stupid question from the only non-astronomer here, but does this mean, conceptually, that we have something like a “critical mass” for nonradioactive elements going into fusion as we do for radioactive ones going into fission?

Only that the critical mass is rather a lot of orders of magnitude more than for radioactive elements?

Except it is neither critical mass for fission, it is “critical slow neutron density” and, for fusion it is to do with a combination of proton energy (i.e. temperature) and collision frequency (i.e. pressure.)

If you want a great big ball of hydrogen to start fusing on its own, you are going to need vast amounts more than say, you need for Plutonium 239. About 10^28 times as much.

Oh, hang on – not all radioactive elements are fissile and not all non-radioactive ones will undergo fusion either.

Only heavier elements are fissile (although few isotopes will do this in a manner that will permit a chain reaction to occur – it is much more common for them to absorb the neutron and then either undergo some form of radioactive decay, often Beta emission, or simply regurgitate a neutron)

All lighter than iron elements are capable of some sort of fusion – often absorption of a hydrogen or helium nucleus. In fusion bombs, the core fusion material is quite highly radioactive tritum (Hydrogen-3).

The article is very interesting, but doesn’t seem very well-written. For example:

(1) “up to seven times the size of Jupiter”

Not clear what that may mean. Seven times the diameter? Seven times the mass?

(2) “it has a temperature of approximately 400 degrees Celsius” and “theorists had established the existence of this type of very cold and young planet”

OK, so is it very hot? Or very cold? Or as the article has it, both?

(3) “we only found one homeless planet in our neighbourhood” and “This nearby free-floating object”

Well, don’t you wonder how big is our “neighbourhood”? That is, how “near” is this massive, free-floating object, really? And anyway, how “free-floating” is it, if it’s part of a start cluster? Most importantly, how likely to float even nearer to us?

Shouldn’t such questions occur to a science reporter? Especially since they occur to some doofus like me in a weblog comment?

2. 400C is 670ishK which is very hot for space but very cold for a proto-star. It is pretty cold for a planet too, with the caveat that many exoplanets currently known are very close in to their stars (and therefore hot). Simply because both the wobble and occultation methods for detecting planets work best the larger the planet and the closer to the star.

3. 31 parsecs with an error of about 30%, according to the paper. On the order of 100 light years. If they have the association correctly. Not likely to be a problem to the human race … (And, technically, the ABDMG is a bit too small to be considered a ‘star cluster’.)

I don’t know. If it hit Jupiter, it could be kind of spectacular. But the problem is we are not ready for it yet. If we were, we could put something nice in orbit around it and send it off. Like a human settlement. Or if we were really really prepared for it, we could build a concrete shell around the entire planet, about where gravity would be one G, add some water and an atmosphere, and so help seed the life through out the universe. Slowly.

We don’t have artificial gravity. What would hold people down? Unless you built it close enough to the sun that there would be exactly 1 g on the other surface. But that would be the outer surface. And the engineering challenges might be a little big.

The engineering challenges wouldn’t necessarily be of any order larger than building your concrete shell around this wanderer. For a start, we’ve got to get our construction crews 100 light years. I’ll take a bet that we’re unlikely to live to have to pay out that we have artificial gravity long before the first man-made object gets 100ly from Earth.

Let’s be honest – if we were ready to react to something 7 times the mass of Jupiter entering the solar system, the world would be a very different place.

Surreptitious Evil – “The engineering challenges wouldn’t necessarily be of any order larger than building your concrete shell around this wanderer.”

Sorry but yes they would. My plan is open to a simple brute force approach. A lot of concrete and a lot of energy to throw it energetically about the place. Artificial gravity means entirely new physics and engineering that we cannot even begin to outline now. That is one or two orders of magnitude larger. It is the difference between, say, the Three Gorges Dam on a massive scale and producing two or three Einsteins plus a couple of Von Neumans.

“For a start, we’ve got to get our construction crews 100 light years. I’ll take a bet that we’re unlikely to live to have to pay out that we have artificial gravity long before the first man-made object gets 100ly from Earth.”

That may be a little difficult but actually we do know how to do that already – Project Orion. Cheaply too. We have to want it. I agree with you that we will not likely see a man-made object get 100 ly from the Earth. Because we face a long slow slide back into the Dark Ages from which it is unlikely the human race will emerge. But in terms of engineering one is a lot harder than the other.

“Let’s be honest – if we were ready to react to something 7 times the mass of Jupiter entering the solar system, the world would be a very different place.”